Photovoltaic devices, or solar cells, respectively, are devices which convert light, especially sunlight, into electric energy. For low-cost mass production, thin film solar cells are of interest since they allow using glass, glass ceramics or other rigid or flexible substrates instead of crystalline or polycrystalline silicon as a base material, or substrate, respectively. With respect to thin-film solar cells, a sequence of thin, partially doped silicon or silicon alloy films may be arranged, sandwiched between transparent conductive electrodes on a carrier substrate, for example glass, plastic, or steel.
Various solar cell technologies are commercially available today. The possibility to process such cells at low temperatures and in a large scale is a major advantage of this technology.
The solar cell structure, or the layer sequence, respectively, is responsible for or capable of the photovoltaic effect. The layers may be deposited, as thin layers by means of known vacuum deposition techniques such as PVD, CVD, PECVD, and/or APCVD, all of which may be used in semiconductor technology.
Conventional thin-film solar cells usually comprise a transparent electrode layer, also called front electrode, deposited on a substrate. On top of this first electrode layer, a photoelectric conversion semiconductor layer formed of a thin amorphous and/or microcrystalline silicon film and a back electrode layer are usually deposited. Said back electrode may again comprise a transparent conductive layer as well as a reflector layer, a conductive and reflective metal layer or a technical equivalent thereof.
In detail, prior Art FIG. 1 shows a basic, simple photovoltaic cell 10 comprising a transparent substrate 12, with a layer of a transparent conductive oxide (TCO) 14, for example formed of zinc oxide (ZnO), or tin oxide (SnO2), deposited thereon. This layer is also called front contact and acts as first electrode for the photovoltaic element. The combination of substrate 12 and front contact 14 is also known as superstrate. The next layer 16 acts as the active photovoltaic layer and exhibits three “sub-layers” forming a p-i-n junction. Said layer 16 comprises hydrogenated microcrystalline, nanocrystalline or amorphous silicon or a combination thereof. Sub-layer 18, which is arranged adjacent to the TCO front contact 14, is positively doped, the adjacent sub-layer 20 is intrinsic, and the final sub-layer 22 is negatively doped. In an alternative embodiment, the layer sequence p-i-n as described can be inverted to n-i-p. In this case, layer 16 is identified as n-layer, layer 20 again as intrinsic, layer 22 as p-layer.
Finally, the cell includes a rear contact layer 24, which also is called back contact. Additionally, a reflective layer 26 is provided. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 26 and back contact 24. For illustrative purposes, arrows indicate impinging light.
Zinc oxide, which may preferably be deposited by low pressure CVD (LPCVD), is widely used for front and back contacts due to the fact that it exhibits outstanding properties in terms of haze due to its surface roughness. Zinc oxide thus enhances the light trapping properties of a solar cell.
However, zinc (Zn), or additionally comparable metals such as tin (Sn) or indium (In), or the respective oxides also may have a negative effect on the performance of photovoltaic modules. In detail, during deposition of subsequent silicon layers by PECVD, for example, it is difficult to avoid reduction of the oxygen compounds of zinc, tin or indium by the PECVD hydrogen plasma. Generally, this effect may appear at all metal-oxide compounds usable for front contacts.
This disadvantage is mainly caused by the fact that hydrogen is commonly used in plasma assisted deposition of silicon. In the case of the widely used zinc oxide, for example, zinc is then in some cases deposited and thus stored at the interior of the reactor, i.e. in its chamber, such as on the walls or on other components of the reactor. It thus forms contamination at the interior of the reactor. Zinc or the aforementioned metals as well as their oxides stored in the reactor will then slowly be released during subsequent deposition steps due to the comparatively large vapour pressure. Due to this slow release of the contaminants during deposition of silicon, the contaminants, or the contamination, respectively, are embedded in the deposited silicon layers resulting in the module performance of the so formed photovoltaic cell being reduced.
It is thus required to remove the contamination from the interior of the reactor, i.e. the reactor chamber. In detail, a method to remove contamination from the reactor, or its interior, respectively, would allow improving the efficiency and the light stability of solar cells formed in the reactor.
Known methods to remove a contaminant, such as zinc, usually involve opening the contaminated reactor and cleaning the interior surfaces with cleaning agents, solvents or other chemical compounds. Such solutions are time consuming and expensive.
A further possible way to reduce the amount of zinc accumulated in a reactor involves depositing solar cells on glass without a TCO front contact and throwing away the glasses. This process, however, is effective but costly and time consuming.